Glass fiber reinforced resin tanks are typically made of isophthalic resin, chopped glass strands, and a filler such as sand. External ribs of woven glass roving and glass filament windings impregnated with resin are usually provided. Generally a resin-rich liner is also provided consisting of about a 20-mil thickness of resin with a surface mat and an 80-mil thickness of resin and chopped glass strands. When a tank is to be used to store corrosive liquids which attack isophthalic resin, polyester or vinylester resin may be used as the liner resin. The tanks are made of two halves, as generally disclosed in U.S. Pat. No. 3,700,512, joined by an external lay-up of a plurality of interleaved layers of woven glass roving and chopped strand mat impregnated with resin. Prior to my invention, tanks for corrosive liquid service were also provided at the joint with an internal lay-up of a plurality of interleaved layers of woven glass roving and chopped strand mat impregnated with a corrosion-resistant resin such as polyester or vinylester resin. For application of this internal lay-up, one of the tank halves had to be provided with a manway, which added considerably to the cost of the tank.